In Italy a few months ago, I bought several packs of dried porcini mushrooms. When I soaked them in warm water, tiny worms floated to the surface. The package had been sealed so where did the worms come from? Unfortunately, the warm, moist environment mushrooms grow in makes them attractive to bugs seeking to incubate their eggs. This is not usually a problem with cultivated mushrooms, which are grown under controlled conditions, but it is with wild varieties. As you found, the drying process does not kill the bugs. When buying mushrooms, inspect them for tiny holes (this is difficult with mushrooms in packs). It is best to store the mushrooms in the freezer so if there are any eggs (as long as I do not see them, I can pretend they do not exist) they will not have a chance to hatch. If you are not squeamish about this kind of thing and the infestation does not seem bad, pour water over the mushrooms and let the bugs float to the surface. Pour the water (and the bugs) away and soak the mushrooms until soft in fresh water. I also freeze other food products that seem susceptible to bugs, such as wheat flour. Can you substitute butter for shortening in recipes? It depends on the recipe. Shortening is pure fat, made with hydrogenised, solid vegetable oils. Butter is composed of 80 per cent butter fat (better butters contain a higher percentage), and 20 per cent milk solids and water. If you try to use butter for deep-frying the non-fat elements will burn. Clarified butter, which has had the water and milk solids removed, can be used for frying. When baking, you can use butter in recipes that call for shortening. I never use shortening because butter has more flavour. Shortening is popular in pie doughs because it gives a flaky texture. If you want a flaky-textured pie with a buttery taste, try using half butter and half shortening. I still prefer to use all butter in my pie doughs, despite the change in texture. In cookies and cakes, you can substitute butter for shortening in equal proportions. The flavour will be much better.